Ionic liquid and power storage device including the same

ABSTRACT

An ionic liquid having high electrochemical stability and a low melting point. An ionic liquid represented by the following general formula (G0) is provided. 
                         
In the general formula (G0), R 0  to R 5  are individually any of an alkyl group having 1 to 20 carbon atoms, a methoxy group, a methoxymethyl group, a methoxyethyl group, and a hydrogen atom, and A −  is a univalent imide-based anion, a univalent methide-based anion, a perfluoroalkyl sulfonic acid anion, tetrafluoroborate, or hexafluorophosphate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an ionic liquid and a power storagedevice including the ionic liquid.

2. Description of the Related Art

Owing to the increase in demand for portable electronic devices such asa mobile phone and a laptop personal computer and the development ofelectric vehicles (EVs), the demand for power storage devices such as anelectric double layer capacitor, a lithium-ion secondary battery, and alithium-ion capacitor has been significantly increasing in recent years.For the power storage devices, high capacity, high performance, safetyin various operating environments, and the like are required.

To satisfy the above requirements, electrolyte solutions for powerstorage devices have been actively developed. Cyclic carbonates are usedfor electrolyte solutions for power storage devices. In particular,ethylene carbonate is often used because of its high dielectric constantand high ionic conductivity.

However, not only an ethylene carbonate but also many organic solventshave volatility and a low flash point. For this reason, in the casewhere an organic solvent is used for an electrolyte solution of a powerstorage device, the temperature inside a lithium-ion secondary batterymight rise due to a short circuit, overcharge, or the like and thelithium-ion secondary battery might burst or catch fire.

In consideration of the risks, use of an ionic liquid, which isnonvolatile and flame-retardant, for an electrolyte solution of a powerstorage device has been studied. An ionic liquid is also referred to asambient temperature molten salt, which is a salt formed by combinationof cations and anions. Examples of the ionic liquid include an ionicliquid including quaternary ammonium-based cations and an ionic liquidincluding imidazolium-based cations (see Patent Document 1 andNon-Patent Document 1).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2003-331918

Non-Patent Document

-   Hajime Matsumoto, et al., Fast cycling of Li/LiCoO₂ cell with    low-viscosity ionic liquids based on bis(fluorosulfonyl)imide    [FSI]⁻, Journal of Power Sources 160, 2006, pp. 1308-1313

SUMMARY OF THE INVENTION

When an ionic liquid is used for an electrolyte solution of a powerstorage device, the ionic liquid needs to have a wide potential windowand high electrochemical stability. Furthermore, the operatingtemperature of a power storage device ranges from −30° C. to 70° C. inmany cases. In order to use the power storage device in variousenvironments, particularly in a low-temperature environment, the ionicliquid preferably has a low melting point.

Patent Document 1 discloses an ionic liquid including quaternaryammonium-based cations which has a melting point of approximately 10° C.When the power storage device including such an ionic liquid is used ina low-temperature environment, there is a possibility that the ionicliquid solidifies and the resistance of the ionic liquid increases. Inaddition, a problem in which the range of operating temperature of thepower storage device is narrowed occurs when it is difficult to use thepower storage device in a low-temperature environment.

Furthermore, as described in Non-Patent Document 1, an ionic liquidincluding imidazolium-based cations has a narrower potential window andthus lower electrochemical stability than an ionic liquid includingquaternary ammonium-based cations. Therefore, the ionic liquid includingimidazolium-based cations is unstable to a positive-electrode materialand a negative-electrode material; accordingly, the reliability of thepower storage device might be decreased.

In view of the above problems, an object of an embodiment of the presentinvention is to provide an ionic liquid having high electrochemicalstability and a low melting point. Another object is to provide a powerstorage device an electrolyte solution of which includes the ionicliquid.

An embodiment of the present invention is an ionic liquid including acyclic quaternary ammonium cation and a univalent anion. The ionicliquid is represented by the following general formula (G0).

In the general formula (G0), R⁰ to R⁵ are individually any of an alkylgroup having 1 to 20 carbon atoms, a methoxy group, a methoxymethylgroup, a methoxyethyl group, and a hydrogen atom, and A⁻ is a univalentimide-based anion, a univalent methide-based anion, a perfluoroalkylsulfonic acid anion, tetrafluoroborate (BF₄ ⁻), or hexafluorophosphate(PF₆ ⁻).

Another embodiment of the present invention is an ionic liquid includinga cyclic quaternary ammonium cation and a univalent anion. The ionicliquid is represented by the following general formula (G1).

In the general formula (G1), one or two of R¹ to R⁴ are any of an alkylgroup having 1 to 20 carbon atoms, a methoxy group, a methoxymethylgroup, and a methoxyethyl group. The other two or three of R¹ to R⁴ arehydrogen atoms, and A⁻ is a univalent imide-based anion, a univalentmethide-based anion, a perfluoroalkyl sulfonic acid anion, BF₄ ⁻, or PF₆⁻.

In the general formula (G1), it is preferable that one or two of R¹ toR⁴ be each an alkyl group having 1 to 4 carbon atoms.

Another embodiment of the present invention is an ionic liquid includinga cyclic quaternary ammonium cation and a univalent anion. The ionicliquid is represented by the following general formula (G2).

In the general formula (G2), R¹ and R² are individually any of an alkylgroup having 1 to 20 carbon atoms, a methoxy group, a methoxymethylgroup, and a methoxyethyl group, and A⁻ is a univalent imide-basedanion, a univalent methide-based anion, a perfluoroalkyl sulfonic acidanion, BF₄ ⁻, or PF₆ ⁻.

In the general formula (G2), it is preferable that R¹ and R²individually be an alkyl group having 1 to 4 carbon atoms.

Another embodiment of the present invention is an ionic liquid includinga cyclic quaternary ammonium cation and a univalent anion. The ionicliquid is represented by the general formula (G3).

In the general formula (G3), A⁻ is a univalent imide-based anion, aunivalent methide-based anion, a perfluoroalkyl sulfonic acid anion, BF₄⁻, or PF₆ ⁻.

Another embodiment of the present invention is an ionic liquid includinga cyclic quaternary ammonium cation and a univalent anion. The ionicliquid is represented by the general formula (G4).

In the general formula (G4), A⁻ is a univalent imide-based anion, aunivalent methide-based anion, a perfluoroalkyl sulfonic acid anion, BF₄⁻, or PF₆ ⁻.

Another embodiment of the present invention is an ionic liquid includinga cyclic quaternary ammonium cation and a univalent anion. The ionicliquid is represented by the general formula (G5).

In the general formula (G5), A⁻ is a univalent imide-based anion, aunivalent methide-based anion, a perfluoroalkyl sulfonic acid anion, BF₄⁻, or PF₆ ⁻.

In the general formulae (G0) to (G5), A⁻ is preferably any one ofunivalent anions selected from (C_(n)F_(2n+1)SO₂)₂N⁻ (n=0 to 4),(C_(m)F_(2m+1)SO₃)⁻ (m=0 to 4), and CF₂(CF₂SO₂)₂N⁻.

Another embodiment of the present invention is a power storage deviceincluding a positive electrode, a negative electrode, a separator, andan electrolyte solution. The electrolyte solution includes an ionicliquid represented by any of the general formulae (G0) to (G5).

Another embodiment of the present invention is a power storage deviceincluding a positive electrode, a negative electrode, a separator, andan electrolyte solution. The electrolyte solution includes an ionicliquid represented by any of the general formulae (G0) to (G5) andelectrolyte salt including a lithium ion.

Note that the power storage device in this specification indicates allelements and devices which have the function of storing power. Forexample, a lithium-ion secondary battery, a lithium-ion capacitor, andan electric double layer capacitor are included in the category of thepower storage device.

According to an embodiment of the present invention, an ionic liquidhaving high electrochemical stability and a low melting point can beprovided. Further, a power storage device an electrolyte solution ofwhich includes an ionic liquid according to an embodiment of the presentinvention can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are cross-sectional views illustrating power storagedevices;

FIGS. 2A and 2B are a top view and a perspective view illustrating powerstorage devices;

FIGS. 3A and 3B are perspective views illustrating a method formanufacturing a power storage device;

FIG. 4 is a perspective view illustrating a method for manufacturing apower storage device;

FIGS. 5A and 5B illustrate application examples of a power storagedevice;

FIGS. 6A and 6B illustrate application examples of a power storagedevice;

FIG. 7 is an NMR chart of 1,3-dimethyl-1-propylpyrrolidiniumbis(trifluoromethanesulfonyl)amide (abbreviation: 3mP13-TFSA);

FIG. 8 is an NMR chart of 1,3-dimethyl-1-propylpyrrolidiniumbis(fluorosulfonyl)amide (abbreviation: 3mP13-FSA);

FIG. 9 is an NMR chart of 1,2-dimethyl-1-propylpyrrolidiniumbis(fluorosulfonyl)amide (abbreviation: 2mP13-FSA);

FIG. 10 shows linear sweep voltammograms of Samples 1 to 3 andComparative Samples 1 to 3;

FIG. 11 is a top view illustrating an electric double layer capacitor;

FIG. 12A is a graph showing the solution resistances of Sample 4 andComparative Sample 4 and FIG. 12B is a graph showing the cellresistances thereof;

FIG. 13 is a graph showing output characteristics of Sample 5,Comparative Sample 5, and Comparative Sample 6;

FIG. 14 is an NMR chart of 1-ethyl-1,3-dimethylpyrrolidiniumbis(fluorosulfonyl)amide (abbreviation: 3mP12-FSA);

FIG. 15 shows a linear sweep voltammogram of 3mP12-FSA;

FIG. 16 is a perspective view illustrating a lithium-ion secondarybattery; and

FIG. 17A is a graph showing output characteristics of Sample 6 and FIG.17B is a graph showing output characteristics of Sample 7.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. Note that thepresent invention is not limited to the description below, and it iseasily understood by those skilled in the art that a variety of changesand modifications can be made without departing from the spirit andscope of the present invention. Therefore, the present invention is notconstrued as being limited to the description of the embodiments andexamples given below.

Embodiment 1

An embodiment of the present invention is an ionic liquid including acyclic quaternary ammonium cation and a univalent anion. The ionicliquid is represented by the following general formula (G0).

In the general formula (G0), R⁰ to R⁵ are individually any of an alkylgroup having 1 to 20 carbon atoms, a methoxy group, a methoxymethylgroup, a methoxyethyl group, and a hydrogen atom, and A⁻ is a univalentimide-based anion, a univalent methide-based anion, a perfluoroalkylsulfonic acid anion, BF₄ ⁻, or PF₆ ⁻.

Another embodiment of the present invention is an ionic liquid includinga cyclic quaternary ammonium cation and a univalent anion. The ionicliquid is represented by the following general formula (G1).

In the general formula (G1), one or two of R¹ to R⁴ are any of an alkylgroup having 1 to 20 carbon atoms, a methoxy group, a methoxymethylgroup, and a methoxyethyl group. The other two or three of R¹ to R⁴ arehydrogen atoms, and A⁻ is a univalent imide-based anion, a univalentmethide-based anion, a perfluoroalkyl sulfonic acid anion, BF₄ ⁻, or PF₆⁻.

In the general formula (G1), it is preferable that one or two of R¹ toR⁴ be each alkyl group having 1 to 4 carbon atoms.

Another embodiment of the present invention is an ionic liquid includinga cyclic quaternary ammonium cation and a univalent anion. The ionicliquid is represented by the following general formula (G2).

In the general formula (G2), R¹ and R² are individually any of an alkylgroup having 1 to 20 carbon atoms, a methoxy group, a methoxymethylgroup, and a methoxyethyl group, and A⁻ is a univalent imide-basedanion, a univalent methide-based anion, a perfluoroalkyl sulfonic acidanion, BF₄ ⁻, or PF₆ ⁻.

In the general formula (G2), it is preferable that R¹ and R²individually be an alkyl group having 1 to 4 carbon atoms.

Another embodiment of the present invention is an ionic liquid includinga cyclic quaternary ammonium cation and a univalent anion. The ionicliquid is represented by the following general formula (G3).

In the general formula (G3), A⁻ is a univalent imide-based anion, aunivalent methide-based anion, a perfluoroalkyl sulfonic acid anion, BF₄⁻, or PF₆ ⁻.

Another embodiment of the present invention is an ionic liquid includinga cyclic quaternary ammonium cation and a univalent anion. The ionicliquid is represented by the following general formula (G4).

In the general formula (G4), A⁻ is a univalent imide-based anion, aunivalent methide-based anion, a perfluoroalkyl sulfonic acid anion, BF₄⁻, or PF₆ ⁻.

Another embodiment of the present invention is an ionic liquid includinga cyclic quaternary ammonium cation and a univalent anion. The ionicliquid is represented by the general formula (G5).

In the general formula (G5), A⁻ is a univalent imide-based anion, aunivalent methide-based anion, a perfluoroalkyl sulfonic acid anion, BF₄⁻, or PF₆ ⁻.

Specific examples of the cyclic quaternary ammonium cation includeorganic compounds represented by the structural formulae (100) to (117).Note that embodiments of the present invention are not limited thereto.

Here, calculation results of an improvement of the reduction resistancedue to an electron donating substituent are shown.

As a cation included in an ionic liquid according to an embodiment ofthe present invention, 1,2-dimethyl-1-propylpyrrolidinium cation(abbreviation: 2mP13) represented by the structural formula (100).1,3-dimethyl-1-propylpyrrolidinium cation (abbreviation: 3mP13)represented by the structural formula (101), and1-ethyl-1,3-dimethylpyrrolidinium cation (abbreviation: 3mP12)represented by the structural formula (110) are shown below. As a cationincluded in a comparative ionic liquid, 1-methyl-1-propylpyrrolidiniumcation (abbreviation: P13) represented by the structural formula (301),and 1-ethyl-3-methylimidazolium cation (abbreviation: EMI) representedby the structural formula (302) are shown below.

In the quantum chemistry computation, the optimal molecular structuresin the singlet excited state of cations each included in an ionic liquidaccording to an embodiment of the present invention (structural formulae(100), (101), and (110)) and cations each included in the comparativeionic liquid (structural formulae (301) and (302)) are calculated byusing density functional theory (DFT). In the DFT, the total energy isrepresented as the sum of potential energy, electrostatic energy betweenelectrons, electronic kinetic energy, and exchange-correlation energyincluding all the complicated interactions between electrons. In theDFT, an exchange-correlation interaction is approximated by a functional(that is, a function of another function) of one electron potentialrepresented in terms of electron density to enable high-speed andhighly-accurate calculations. Here. B3LYP, which is a hybrid functional,is used to specify the weight of each parameter related toexchange-correlation energy. In addition, as a basis function, 6-311G (abasis function of a triple-split valence basis set using threecontraction functions for each valence orbital) is applied to all theatoms. By the above basis function, for example, orbits of is to 3s areconsidered in the case of hydrogen atoms while orbits of 1s to 4s and 2pto 4p are considered in the case of carbon atoms. Furthermore, toimprove calculation accuracy, the p function and the d function aspolarization basis sets are added to hydrogen atoms and atoms other thanhydrogen atoms, respectively.

Note that Gaussian 09 was used as a quantum chemistry computationalprogram. The calculations were performed using a high performancecomputer (Altix ICE8400EX manufactured by SGI). The quantum chemistrycomputation was performed on the assumption that all of the cationsrepresented by the structural formulae (100), (101), (110), (301), and(302) included in ionic liquids have the most stable structure and werein a vacuum.

Table 1 shows the lowest unoccupied molecular orbital levels (LUMOlevels), which are calculated by the quantum chemistry computation, ofthree types of cations represented by the structural formulae (100),(101), and (110) included in ionic liquids. For comparison, Table 1 alsoshows the lowest unoccupied molecular orbital levels (LUMO levels) oftwo types of cations represented by the structural formulae (301) and(302) in ionic liquids.

TABLE 1 LUMO level [eV] Structural formula (100) −3.39 Structuralformula (101) −3.47 Structural formula (110) −3.31 Structural formula(301) −3.50 Structural formula (302) −5.11

When an ionic liquid is used for an electrolyte solution of a powerstorage device, the reduction resistance of the ionic liquid depends onthe electron acceptability of a cation in the ionic liquid from anegative electrode.

For example, when the LUMO level of a cation included in the ionicliquid is higher than a conduction band of a negative-electrodematerial, the ionic liquid including the cation is not reduced. Thereduction resistance of the cation with respect to lithium can berelatively evaluated by comparing the LUMO level of the cation with theLUMO level of the EMI cation represented by the structural formula (302)having a reduction potential substantially equivalent to anoxidation-reduction potential of lithium that is a typical low potentialnegative electrode material. In other words, when a cation included inan ionic liquid according to an embodiment of the present invention hasa higher LUMO level than at least an EMI cation represented by thestructural formula (302), the ionic liquid according to the embodimentof the present invention has excellent reduction resistance.

As shown in Table 1, a P13 cation represented by the structural formula(301) has a LUMO level of −3.50 eV, while a 2mP13 cation represented bythe structural formula (100), a 3mP13 cation represented by thestructural formula (101), and a 3mP12 cation represented by thestructural formula (110) each have a LUMO level higher than −3.50 eV.Therefore, the ionic liquids according to the embodiments of the presentinvention have excellent reduction resistance.

The above calculation results indicate that the reduction resistance ofan ionic liquid is improved by introduction of an electron donatingsubstituent to a cation of the ionic liquid.

In the general formulae (G0) to (G5), A⁻ is preferably any one ofunivalent anions selected from (C_(n)F_(2n+1)SO₂)₂N⁻ (n=0 to 4),(C_(m)F_(2m+1)SO₃)⁻ (m=0 to 4), and CF₂(CF₂SO₂)₂N⁻. Note that A⁻ is notlimited thereto and may be any anion as long as it pairs with a cyclicquaternary ammonium cation to form an ionic liquid.

The oxidation potential of the ionic liquid differs depending on typesof anions. When any one of univalent anions selected from(C_(n)F₂₊₁SO₂)₂N⁻ (n=0 to 4), (C_(m)F_(2m+1)SO₃)⁻ (m=0 to 4), andCF₂(CF₂SO₂)₂N⁻ is used, the oxidation potential of the ionic liquid canbe high due to mutual interaction with the above anion. This means thatthe oxidation resistance of the ionic liquid can be improved.

Next, a method for synthesizing an ionic liquid according to anembodiment of the present invention is described. A variety of reactionscan be applied to the method for synthesizing the ionic liquid accordingto an embodiment of the present invention. For example, an ionic liquidrepresented by the general formula (G1) can be synthesized by asynthesis method described below. Note that the method for synthesizingthe ionic liquid according to an embodiment of the present invention isnot limited to the following synthesis methods.

<Method for Synthesizing Ionic Liquid Represented by General Formula(G1)>

Various types of reactions can be applied to the method for synthesizingan ionic liquid according an embodiment of the present invention. Here,an example is described referring to the synthesis scheme (S-1).

In the synthesis scheme (S-1), a reaction from the general formula (α-1)to the general formula (α-2) is a ring closure reaction of amino alcoholwhich passes through halogenation using trisubstituted phosphine such astrialkylphosphine and halogen source. PR′ represents trisubstitutedphosphine and X₁ represents halogen source. As the halogen source,carbon tetrachloride, carbon tetrabromide, iodine, or iodomethane can beused, for example. In the ionic liquid according to an embodiment of thepresent invention, triphenylphosphine is used as the trisubstitutedphosphine and carbon tetrachloride is used as the halogen source.

In the above scheme (S-1), the reaction from the general formula (α-2)to the general formula (α-3) is alkylation of amine by an amine compoundand a carbonyl compound in the presence of hydride. For example,excessive formic acid can be used as the hydride source. CH₂O is used asthe carbonyl compound in the ionic liquid according to an embodiment ofthe present invention.

In the above scheme (S-1), the reaction from the general formula (α-3)to the general formula (α-4) is alkylation by a tertiary amine compoundand an alkyl halide compound, which synthesizes quaternary ammoniumsalt. As the alkyl halide compound, propane halide or bromoethane can beused. Further, X₂ represents halogen. The halogen is preferably bromine,more preferably iodine, in terms of high reactivity.

Through ion exchange between the quaternary ammonium salt represented bythe general formula (α-4) and desired metal salt, the ionic liquidrepresented by the general formula (G1) can be obtained. As the metalsalt, lithium metal salt can be used, for example.

<Method for Synthesizing Ionic Liquid Represented by General Formula(G4)>

Next, a method for synthesizing an ionic liquid according an embodimentof the present invention is described referring to the synthesis scheme(S-2).

In the above scheme (S-2), the reaction from the general formula (α-5)to the general formula (α-6) is alkylation of amine by an amine compoundand a carbonyl compound in the presence of hydride. For example,excessive formic acid can be used as the hydride source. CH₂O is used asthe carbonyl compound in the ionic liquid according to an embodiment ofthe present invention.

In the above scheme (S-2), the reaction from the general formula (α-6)to the general formula (α-7) is alkylation by a tertiary amine compoundand an alkyl halide compound, which synthesizes quaternary ammoniumsalt. As the alkyl halide compound, propane halide or bromoethane can beused. Further, X₃ represents halogen. The halogen is preferably bromine,more preferably iodine, in terms of high reactivity.

Through ion exchange between the quaternary ammonium salt represented bythe general formula (α-7) and desired metal salt, the ionic liquidrepresented by the general formula (G4) can be obtained. As the metalsalt, lithium metal salt can be used, for example.

The ionic liquid according to the embodiment of the present inventionhas a wide potential window greater than or equal to 0.2 and less thanor equal to 5.4, preferably greater than or equal to −0.1 and less thanor equal to 5.8. That is, the ionic liquid according to the embodimentof the present invention has high electrochemical stability.

Furthermore, the melting point of the ionic liquid can be decreased byintroduction of a substituent to the cyclic quaternary ammonium. Forexample, the melting point can be decreased by introduction of a methylgroup to a pyrrolidine skeleton. The melting point of the ionic liquidaccording to the embodiment of the present invention can be less than orequal to −10° C., preferably less than or equal to −30° C. The reasonfor this decrease in melting point is that the symmetry of the cyclicquaternary ammonium molecule is broken by the introduction of asubstituent to the cyclic quaternary ammonium.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 2

In this embodiment, a power storage device in which the ionic liquidaccording to an embodiment of the present invention is used for anelectrolyte solution is described with reference to FIGS. 1A and 1B,FIGS. 2A and 2B, FIGS. 3A and 3B, and FIG. 4.

The power storage device according to an embodiment of the presentinvention includes at least a positive electrode, a negative electrode,a separator, and an electrolyte solution. For example, an electrolytesolution used for a lithium-ion secondary battery or a lithium-ioncapacitor includes a nonaqueous solvent and electrolyte salt. The ionicliquid according to an embodiment of the present invention can be usedfor the nonaqueous solvent in which the electrolyte salt dissolves.Alternatively, only the ionic liquid according to an embodiment of thepresent invention can be used for an electrolyte solution in an electricdouble layer capacitor without use of the electrolyte salt.

FIG. 1A illustrates a structure of a power storage device 100. The casewhere the power storage device 100 is a lithium-ion secondary battery isdescribed below as an example.

The power storage device 100 illustrated in FIG. 1A includes a positiveelectrode 103 including a positive electrode current collector 101 and apositive electrode active material layer 102, a negative electrode 106including a negative electrode current collector 104 and a negativeelectrode active material layer 105, a separator 107, an electrolytesolution 108, and a housing 109. The separator 107 is placed between thepositive electrode 103 and the negative electrode 106 provided in thehousing 109. The inside of the housing 109 is filled with theelectrolyte solution 108.

For the positive electrode current collector 101, a conductive materialcan be used, for example, Examples of the conductive material includealuminum (Al), copper (Cu), nickel (Ni), and titanium (Ti). In addition,an alloy material including two or more of the above-mentionedconductive materials can be used for the positive electrode currentcollector 101. Examples of the alloy material include an Al—Ni alloy andan Al—Cu alloy. The positive electrode current collector 101 can have,for example, a foil shape, a plate shape, or a net shape as appropriate.Further, the positive electrode current collector 101 can be formed insuch a manner that a conductive layer is formed over another formationsubstrate, and the conductive layer is separated from the formationsubstrate.

For the positive electrode active material layer 102, a materialincluding ions to serve as carriers and a transition metal can be used,for example. As the material including ions to serve as carriers and atransition metal, a material represented by a general formulaA_(h)M_(i)PO_(j) (h>0, i>0, j>0) can be used, for example. Here, Arepresents, for example, an alkali metal such as lithium, sodium, orpotassium; or an alkaline earth metal such as calcium, strontium, orbarium; beryllium; or magnesium. M represents, for example, a transitionmetal such as iron, nickel, manganese, or cobalt. Examples of thematerial represented by the general formula A_(h)M_(i)PO_(j) (h>0, i>0,j>0) include lithium iron phosphate and sodium iron phosphate. Thematerial represented by A and the material represented by M may beselected from one or more of the above materials.

Alternatively, a material represented by the general formulaA_(h)M_(i)O_(j) (h>0, i>0, j>0) can be used. Here, A represents, forexample, an alkali metal such as lithium, sodium, or potassium; analkaline earth metal such as calcium, strontium, or barium; beryllium;or magnesium. M represents, for example, a transition metal such asiron, nickel, manganese, or cobalt. Examples of the material representedby the general formula A_(h)M_(i)O_(j) (h>0, i>0, j>0) include lithiumcobaltate, lithium manganate, and lithium nickelate. The materialrepresented by A and the material represented by M may be selected fromone or more of the above materials.

In the case where the power storage device 100 is a lithium-ionsecondary battery, a material including lithium is preferably selectedfor the positive electrode active material layer 102. In other words, Ain the above general formulae A_(h)M_(i)PO_(j) (h>0, i>0, j>0) orA_(h)M_(i)O_(j) (h>0, i>0, j>0) is preferably lithium.

Here, the term “active material” refers only to a material related toinsertion and extraction of ions to serve as carriers. Note that in thisspecification and the like, not only the above-described material(material that is literally a “positive electrode active material”) butalso the above-described material including a conduction auxiliaryagent, a binder, and/or the like is referred to as positive electrodeactive material layer 102 in some cases.

Note that the conduction auxiliary agent may be an electron-conductivematerial which does not cause chemical change in the power storagedevice. For example, a carbon-based material such as acetylene black,graphite, or carbon fiber; a metal material such as copper, nickel,aluminum, or silver; and powder, fiber, and the like of mixtures thereofare given.

As the binder, polysaccharides such as starch, carboxymethyl cellulose,hydroxypropyl cellulose, regenerated cellulose, and diacetyl cellulose;vinyl polymers such as polyvinyl chloride, polyethylene, polypropylene,polyvinyl alcohol, polyvinyl pyrrolidone, polytetrafluoroethylene,polyvinylidene difluoride, ethylene-propylene-diene monomer (EPDM)rubber, sulfonated EPDM rubber, styrene-butadiene rubber, butadienerubber, and fluorine rubber; polyether such as polyethylene oxide; andthe like are given.

For the negative electrode current collector 104, a conductive materialcan be used, for example. Examples of the conductive material includealuminum (Al), copper (Cu), nickel (Ni), and titanium (Ti). In addition,an alloy material including two or more of the above-mentionedconductive materials can be used for the negative electrode currentcollector 104. Examples of the alloy material include an Al—Ni alloy andan Al—Cu alloy. The negative electrode current collector 104 can have,for example, a foil shape, a plate shape, or a net shape as appropriate.Further, the negative electrode current collector 104 can be formed insuch a manner that a conductive layer is formed over another formationsubstrate, and the conductive layer is separated from the formationsubstrate.

There is no particular limitation on the material for the negativeelectrode active material layer 105 as long as it is a material withwhich metal can be dissolved/precipitated or a material in/from whichmetal ions can be inserted/extracted. For the negative electrode activematerial layer 105, a lithium metal, a carbon-based material, silicon, asilicon alloy, or tin can be used, for example. As the carbon-basedmaterial in/from which a lithium ion can be inserted/extracted, a finegraphite powder, a graphite fiber, or graphite can be used, for example.

Note that the negative electrode active material layer 105 may bepredoped with lithium. Predoping with lithium may be performed in such amanner that a lithium layer is formed on a surface of the negativeelectrode active material layer 105 by a sputtering method.Alternatively, a lithium foil is provided on the surface of the negativeelectrode active material layer 105, whereby the negative electrodeactive material layer 105 can be predoped with lithium.

The electrolyte solution 108 includes a nonaqueous solvent andelectrolyte salt. For the nonaqueous solvent, one or more of the ionicliquids each according to an embodiment of the present invention can beused. Note that the nonaqueous solvent is not necessarily a singlesolvent of the ionic liquid according to an embodiment of the presentinvention. The nonaqueous solvent may be a mixed solvent in which any ofthe ionic liquids each according to an embodiment of the presentinvention and another ionic liquid are mixed.

As an electrolyte solution of a power storage device has a lowerreduction potential and higher oxidation potential, that is, a wideroxidation-reduction potential window, the number of materials which canbe selected for a positive electrode and a negative electrode can beincreased. Further, as the electrolyte solution has a wideroxidation-reduction potential window, the electrolyte solution can bestable to the selected materials for the positive electrode and thenegative electrode. With the use of the ionic liquid according to anembodiment of the present invention having a wide oxidation-reductionpotential window for the electrolyte solution, the reliability of thelithium-ion secondary battery can be increased.

The electrolyte salt dissolved in the nonaqueous solvent may beelectrolyte salt which includes ions to serve as carriers andcorresponds with the positive electrode active material layer 102. Theelectrolyte salt may be electrolyte salt including an alkali metal ion,an alkaline earth metal ion, a beryllium ion, or a magnesium ion.Examples of the alkali metal ion include a lithium ion, a sodium ion,and a potassium ion. Examples of the alkaline earth metal ion include acalcium ion, a strontium ion, and a barium ion. In the case where amaterial including lithium is used for the positive electrode activematerial layer 102, electrolyte salt including a lithium ion(hereinafter also referred to as electrolyte salt including lithium) canbe selected. In the case where a material including sodium is used forthe positive electrode active material layer 102, electrolyte saltincluding sodium is preferably selected.

Examples of the electrolyte salt including lithium include lithiumchloride (LiCl), lithium fluoride (LiF), lithium perchlorate (LiClO₄),lithium fluoroborate (LiBF₄), LiAsF₆, LiPF₆, and Li(CF₃SO₂)₂N.

As the separator 107, paper, nonwoven fabric, a glass fiber, a syntheticfiber such as nylon (polyimide), vinylon (a polyvinyl alcohol basedfiber), polyester, acrylic, polyolefin, or polyurethane, or the like maybe used. However, a material which does not dissolve in the electrolytesolution 108 should be selected.

As the separator 107, high-molecular compounds based on fluorine-basedpolymer, polyether such as polyethylene oxide and polypropylene oxide,polyolefin such as polyethylene and polypropylene, polyacrylonitrile,polyvinylidene chloride, polymethyl methacrylate, polymethylacrylate,polyvinyl alcohol, polymethacrylonitrile, polyvinyl acetate,polyvinylpyrrolidone, polyethyleneimine, polybutadiene, polystyrene,polyisoprene, and polyurethane, derivatives thereof, cellulose, paper,and nonwoven fabric can be used either alone or in combination.

For the housing 109, a laminate film, a polymer film, a metal film, ametal case, or a plastic case can be used either alone or incombination.

Next, a power storage device 110, which has a different structure fromthe power storage device 100 illustrated in FIG. 1A, is described withreference to FIG. 1B.

The power storage device 110 illustrated in FIG. 1B includes a positiveelectrode 103 including a positive electrode current collector 101 and apositive electrode active material layer 102, a negative electrode 106including a negative electrode current collector 104 and a negativeelectrode active material layer 105, a separator 107, an electrolytesolution, and a housing 109, which is the same as the power storagedevice 100 illustrated in FIG. 1A. In the power storage device 110illustrated in FIG. 1B, the separator 107, which is placed between thepositive electrode 103 and the negative electrode 106 provided in thehousing 109, is impregnated with the electrolyte solution.

Materials for the positive electrode current collector 101, the positiveelectrode active material layer 102, the negative electrode currentcollector 104, the negative electrode active material layer 105, and thehousing 109 in the power storage device 110 can be the same as those inthe power storage device 100.

In the power storage device 110, the separator 107 is preferably aporous film. As a material for the porous film, a glass fiber, asynthetic resin material, a ceramic material, or the like can be used.In addition, the electrolyte solution with which the separator 107 isimpregnated can be the same as that in the power storage device 100.

Next, a method for manufacturing the power storage device 100 isdescribed. The case where the power storage device 100 is a lithium-ionsecondary battery is described below as an example.

First, the positive electrode active material layer 102 is formed overthe positive electrode current collector 101, so that the positiveelectrode 103 is formed. The positive electrode active material layer102 can be formed by a coating method or a sputtering method using theabove-described material. In the case of employing a coating method forforming the positive electrode active material layer 102, a conductionauxiliary agent, a binder, etc. is mixed with a material including ionsserving as carriers and a transition metal to form a paste, and thepaste is applied onto the positive electrode current collector 101 anddried. In the case of forming the positive electrode active materiallayer 102 by a coating method, pressure forming may also be employed, ifnecessary.

Next, the negative electrode active material layer 105 is formed overthe negative electrode current collector 104, so that the negativeelectrode 106 is formed. The negative electrode active material layer105 can be formed by a coating method, a plasma CVD method, or asputtering method using the above-described material. In the case wheresilicon is used for the negative electrode active material layer 105, amicrocrystalline silicon film may be formed and then amorphous siliconmay be removed from the microcrystalline silicon by etching. The removalof amorphous silicon from microcrystalline silicon, the surface area ofthe remaining microcrystalline silicon is increased. Themicrocrystalline silicon film may be formed by a plasma CVD method or asputtering method. Alternatively, in the case where the negativeelectrode active material layer 105 is formed by a coating method, aconduction auxiliary agent, a binder, etc. is mixed with a material withwhich metal can be dissolved/precipitated or a material in/from whichmetal ions can be inserted/extracted, whereby the negative electrodeactive material layer 105 can be formed in a manner similar to that ofthe positive electrode active material layer 102. Note that theabove-described material can be used as the conduction auxiliary agentand the binder.

In this embodiment, a lithium foil is used for the negative electrode106. Since the ionic liquid according to an embodiment of the presentinvention has excellent reduction resistance and is stable to lithium inthe negative-electrode material that has the lowest potential, with theuse of the ionic liquid for an electrolyte solution, a power storagedevice having a high energy density and high reliability can bemanufactured.

The electrolyte solution 108 and the electrolyte solution with which theseparator 107 is impregnated may be made by mixing any of the ionicliquids in Embodiment 1 and electrolyte salt including a metal ion. Inthis embodiment, Li(CF₃SO₂)₂N is used as the electrolyte salt includinglithium.

The ionic liquid according to an embodiment of the present invention hasa wide oxidation-reduction potential window, and thus is stable to apositive-electrode material and a negative-electrode material.Accordingly, the ionic liquid according to an embodiment of the presentinvention is used for the electrolyte solution 108 and the electrolytesolution with which the separator 107 is impregnated, whereby thereliability of the power storage device can be improved.

Next, the separator 107 is provided between the positive electrode 103and the negative electrode 106 in the housing 109, and the housing 109is filled with the electrolyte solution 108, whereby the power storagedevice 100 can be manufactured. Furthermore, the separator 107impregnated with an electrolyte solution is provided between thepositive electrode 103 and the negative electrode 106 in the housing109, whereby the power storage device 110 can be manufactured.

Next, an example of a laminated power storage device is described withreference to FIG. 2A.

A laminated power storage device 120 illustrated in FIG. 2A includes apositive electrode 103 including a positive electrode current collector101 and a positive electrode active material layer 102, a negativeelectrode 106 including a negative electrode current collector 104 and anegative electrode active material layer 105, a separator 107, anelectrolyte solution 108, and a housing 109. The separator 107 is placedbetween the positive electrode 103 and the negative electrode 106provided in the housing 109. The inside of the housing 109 is filledwith the electrolyte solution 108.

In the power storage device 120 illustrated in FIG. 2A, the positiveelectrode current collector 101 and the negative electrode currentcollector 104 also function as terminals for electrical contact with theoutside. For this reason, each of the positive electrode currentcollector 101 and the negative electrode current collector 104 isprovided to be partly exposed outside the housing 109.

For the housing 109 in the laminated power storage device 120, alaminate film, a polymer film, a metal film, or the like is preferablyused.

Next, an example of a coin-type power storage device is described withreference to FIG. 2B.

A coin-type power storage device 130 illustrated in FIG. 2B includes apositive electrode 103 including a positive electrode current collector101 and a positive electrode active material layer 102, a negativeelectrode 106 including a negative electrode current collector 104 and anegative electrode active material layer 105, a separator 107, anelectrolyte solution 108, and a housing 109, which is the same as thepower storage device 100 illustrated in FIG. 1A. The housing in thepower storage device 130 illustrated in FIG. 2B includes a first housing109 a and a second housing 109 b. In the first housing 109 a and thesecond housing 109 b, the separator 107 impregnated with the electrolytesolution 108 is provided between the positive electrode 103 and thenegative electrode 106.

Next, an example of a method for manufacturing the power storage device130 is described with reference to FIGS. 3A and 3B and FIG. 4.

First, the first housing 109 a is prepared. The first housing 109 a hasa cylinder shape without one bottom surface. A material for the firsthousing 109 a is preferably a conductive material for electricallyconnecting the positive electrode 103 to the outside. The first housing109 a may be formed with, for example, a metal material. In the firsthousing 109 a, the positive electrode 103 including the positiveelectrode current collector 101 and the positive electrode activematerial layer 102 is provided (see FIG. 3A).

On the other hand, the second housing 109 b is prepared. The secondhousing 109 b has a truncated conical shape without a bottom surfacehaving a smaller area than the other bottom surface. A material for thesecond housing 109 b is preferably a conductive material forelectrically connecting the negative electrode 106 to the outside. Thesecond housing 109 b may be formed with, for example, a metal material.In the second housing 109 b, the negative electrode 106 including thenegative electrode current collector 104 and the negative electrodeactive material layer 105 is provided (see FIG. 3B).

FIG. 4 illustrates the first housing 109 a provided with the positiveelectrode 103 including the positive electrode current collector 101 andthe positive electrode active material layer 102; a ring-shapedinsulator 131; the separator 107 impregnated with an electrolytesolution; and the second housing 109 b provided with the negativeelectrode 106 including the negative electrode current collector 104 andthe negative electrode active material layer 105.

The ring-shaped insulator 131 is provided to surround the side of thepositive electrode 103 provided in the first housing 109 a. Thering-shaped insulator 131 has a function of insulating the positiveelectrode 103 from the negative electrode 106. The ring-shaped insulator131 is preferably formed with the use of an insulating resin.

The second housing 109 b provided with the negative electrode 106, whichis illustrated in FIG. 3B, is put in the first housing 109 a providedwith the ring-shaped insulator 131. The separator 107 impregnated withan electrolyte solution is provided between the first housing 109 a andthe second housing 109 b. The second housing 109 b can be fit in thefirst housing 109 a because the diameter of the second housing 109 b issmaller than the diameter of the bottom surface of the first housing 109a. Since the positive electrode 103 and the negative electrode 106 areinsulated from each other with the ring-shaped insulator 131, shortcircuit therebetween does not occur.

Through the above steps, the coin-type power storage device 130illustrated in FIG. 2B can be manufactured.

The ionic liquid according to an embodiment of the present invention hasa wide potential window and high electrochemical stability, and thus isstable to the selected positive-electrode material andnegative-electrode material. Therefore, with the use of the ionic liquidaccording to an embodiment of the present invention for an electrolytesolution of a lithium-ion secondary battery, the reliability of thelithium-ion secondary battery can be increased.

The ionic liquid according to an embodiment of the present invention hasa feature of a low melting point. With the use of the ionic liquidaccording to an embodiment of the present invention for an electrolytesolution of a lithium-ion secondary battery, the lithium-ion secondarybattery can be operated in a low-temperature environment, i.e., theoperating temperature can be widened.

Although the structures of the lithium-ion secondary battery and themanufacturing methods thereof are described with reference to FIGS. 1Aand 1B, FIGS. 2A and 2B, FIGS. 3A and 3B, and FIG. 4, the power storagedevice according to an embodiment of the present invention is notlimited thereto. As another example of the power storage deviceaccording to an embodiment of the present invention, a capacitor isgiven. Examples of the capacitor include a lithium-ion capacitor and anelectric double layer capacitor.

In the case where the power storage devices 100, 110, 120, and 130 areused as lithium-ion capacitors, a material in/from which lithium ionsand/or anions can be reversibly inserted/extracted may be used for thepositive electrode active material layer 102. For the positive electrodeactive material layer 102 and the negative electrode active materiallayer 105, for example, active carbon, graphite, a conductive polymer,or a polyacene organic semiconductor (PAS) can be used.

The ionic liquid according to an embodiment of the present invention hasa wide potential window and high electrochemical stability, and thus isstable to the selected positive-electrode material andnegative-electrode material. Therefore, with the use of the ionic liquidaccording to an embodiment of the present invention for an electrolytesolution of a lithium-ion capacitor, the reliability of the lithium-ioncapacitor can be increased.

The ionic liquid according to an embodiment of the present invention hasa feature of a low melting point. With the use of the ionic liquidaccording to an embodiment of the present invention for an electrolytesolution of a lithium-ion capacitor, the lithium-ion capacitor can beoperated in a low-temperature environment, i.e., the operatingtemperature can be widened.

In the case where the power storage devices 100, 110, 120, and 130 areused as electric double layer capacitors, for the positive electrodeactive material layer 102 and the negative electrode active materiallayer 105, for example, active carbon, a conductive polymer, or apolyacene organic semiconductor (PAS) can be used.

In the case where the power storage devices 100, 110, 120, and 130 areused as electric double layer capacitors, only a nonaqueous solvent canbe used for the electrolyte solution 108 without use of electrolytesalt. For the nonaqueous solvent, one or more of the ionic liquids eachaccording to an embodiment of the present invention can be used.

The ionic liquid according to an embodiment of the present invention hasa wide potential window and high electrochemical stability, and thus isstable to the selected positive-electrode material andnegative-electrode material. Therefore, with the use of the ionic liquidaccording to an embodiment of the present invention for an electrolytesolution of an electric double layer capacitor, the reliability of theelectric double layer capacitor can be increased.

The ionic liquid according to an embodiment of the present invention hasa feature of a low melting point. With the use of the ionic liquidaccording to an embodiment of the present invention for an electrolytesolution of an electric double layer capacitor, the electric doublelayer capacitor can be operated in a low-temperature environment, i.e.,the operating temperature can be widened.

Although examples of a laminated power storage device and a coin-typepower storage device are described in this embodiment, the power storagedevice according to an embodiment of the present invention is notlimited thereto. Various structures can be employed for the powerstorage device; for example, a stack type power storage device or acylinder-type power storage device can be manufactured.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 3

In this embodiment, application of the power storage device according toan embodiment of the present invention is described.

The power storage device according to an embodiment of the presentinvention can be provided in a variety of electronic devices. Examplesof the electronic devices include cameras such as digital cameras orvideo cameras, mobile phones, portable information terminals, e-bookterminals, portable game machines, digital photo frames, and audioreproducing devices. Moreover, the power storage device according to anembodiment of the present invention can be provided in electricallypropelled vehicles such as electric vehicles, hybrid vehicles, electricrailway cars, working vehicles, carts, wheelchairs, and bicycles.

An electrolyte solution of a power storage device according to anembodiment of the present invention includes an ionic liquid accordingto an embodiment of the present invention. The ionic liquid according toan embodiment of the present invention has a wide potential window andhigh electrochemical stability, and thus is stable to the selectedpositive-electrode material and negative-electrode material. Therefore,with the use of the ionic liquid according to an embodiment of thepresent invention for an electrolyte solution of a power storage device,the reliability of the power storage device can be increased.

The ionic liquid according to an embodiment of the present invention hasa feature of a low melting point. With the use of the ionic liquidaccording to an embodiment of the present invention for an electrolytesolution of a lithium-ion secondary battery, the lithium-ion secondarybattery can be operated in a low-temperature environment, i.e., theoperating temperature can be widened.

FIG. 5A illustrates an example of a mobile phone. In a mobile phone3010, a display portion 3012 is incorporated in a housing 3011. Thehousing 3011 is provided with an operation button 3013, an operationbutton 3017, an external connection port 3014, a speaker 3015, amicrophone 3016, and the like. The power storage device according to anembodiment of the present invention is provided in such a mobile phone,whereby the reliability can be improved and the operating temperaturecan be widened. Note that as the power storage device, any one of alithium-ion secondary battery, a lithium-ion capacitor, and an electricdouble layer capacitor or a combination thereof can be used.

FIG. 5B illustrates an example of an e-book terminal. An e-book terminal3030 includes two housings, a first housing 3031 and a second housing3033, which are combined with each other with a hinge 3032. The firsthousing 3031 and the second housing 3033 can be opened and closed withthe hinge 3032 as an axis. A first display portion 3035 and a seconddisplay portion 3037 are incorporated in the first housing 3031 and thesecond housing 3033, respectively. In addition, the second housing 3033is provided with an operation button 3039, a power switch 3043, aspeaker 3041, and the like. The power storage device according to anembodiment of the present invention is provided in such an e-bookterminal, whereby the reliability can be improved and the operatingtemperature can be widened. Note that as the power storage device, anyone of a lithium-ion secondary battery, a lithium-ion capacitor, and anelectric double layer capacitor or a combination thereof can be used.

FIG. 6A illustrates an example of an electric vehicle. An electricvehicle 3050 is equipped with a power storage device 3051. The output ofthe electric power of the power storage device 3051 is controlled by acontrol circuit 3053 and the electric power is supplied to a drivingdevice 3057. The control circuit 3053 is controlled by a computer 3055.

The driving device 3057 includes a DC motor or an AC motor either aloneor in combination with an internal-combustion engine. The computer 3055outputs a control signal to the control circuit 3053 on the basis of aninput data such as data of a driver's operation (e.g., acceleration,deceleration, or stop) or data during driving (e.g., data of an upgradeor a downgrade or data of a load on a driving wheel) of the electricvehicle 3050. The control circuit 3053 adjusts the electric energysupplied from the power storage device 3051 in accordance with thecontrol signal of the computer 3055 to control the output of the drivingdevice 3057. In the case where the AC motor is mounted, an inverterwhich converts direct current into alternate current is incorporated.

Note that as the power storage device 3051, any one of a lithium-ionsecondary battery, a lithium-ion capacitor, and an electric double layercapacitor or a combination thereof can be used. The power storage device3051 can be charged by external electric power supply using a plug-intechnique. The power storage device according to an embodiment of thepresent invention is provided in the electric vehicle, whereby acharging time can be shortened and the convenience can be improved.Furthermore, the reliability can be increased and the operatingtemperature can be widened.

FIG. 6B illustrates an example of an electric wheelchair. A wheelchair3070 includes a control portion 3073 provided with a power storagedevice, a power controller, a control means, and the like. The electricpower of the power storage device is controlled by the control portion3073 to be output and is supplied to a driving portion 3075. The controlportion 3073 is connected to a controller 3077. By operation of thecontroller 3077, the driving portion 3075 can be driven via the controlportion 3073 and speed and movement of the wheelchair 3070 such asmoving forward/backward and a turn can be controlled.

As the power storage device, any one of a lithium-ion secondary battery,a lithium-ion capacitor, and an electric double layer capacitor or acombination thereof can be used. The power storage device can be chargedby external electric power supply using a plug-in technique. When thepower storage device according to an embodiment of the present inventionis provided in the wheelchair 3070, a charging time can be shortened andthe convenience can be improved. Furthermore, the reliability can beincreased and the operating temperature can be widened.

Note that in the case where the power storage device is provided inelectric railway cars as electrically propelled vehicles, the powerstorage device can be charged by power supply from overhead wires orconductive rails.

This embodiment can be combined with any of the structures described inthe other embodiments or examples as appropriate.

Example 1

In this example, a method for producing1,3-dimethyl-1-propylpyrrolidinium bis(trifluoromethanesulfonyl)amide(abbreviation: 3mP13-TFSA) represented by the structural formula (200)is described.

First, 4-amino-2-methyl-1-butanol (24.8 g, 240 mmol) and carbontetrachloride (111 g, 720 mmol) were mixed under a nitrogen atmosphereat room temperature. Into this mixture, triphenylphosphine (69.2 g, 264mmol) dissolved in dehydrated dichloromethane (150 ml) was added.Stirring was performed at 40° C. for 1 to 1.5 hours, and then pure water(50 ml) was added to the reacted solution and stirred well.Subsequently, an aqueous phase and a dichloromethane phase wereseparated. A yellow oily substance was extracted from thedichloromethane phase using pure water (50 ml×2 times). Then, theaqueous phase was washed with toluene (50 ml×3 times), and the solventwas removed by evaporation under reduced pressure to give a yellow oilysubstance.

Sodium hydroxide (19.2 g, 480 mmol) was dissolved in pure water (20 ml),and the sodium hydroxide solution was gradually added to the obtainedyellow oily substance, and the mixture was stirred for 12 hours. Afterthat, distillation was performed to give 3-methylpyrrolidine (18.7 g,219 mmol) which is a colorless transparent liquid.

Into formic acid (21.6 g, 470 mmol) being water-cooled,3-methylpyrrolidine (18.7 g, 219 mmol) was gradually added. Next, a 37%formaldehyde solution (26 ml, 330 mmol) was added to this solution. Thissolution was heated and refluxed at 100° C., was cooled back to roomtemperature after a bubble generation, and was stirred for about 30minutes. Then, the solution was heated and refluxed again for one hour.

The formic acid was neutralized with sodium hydroxide, and then thetarget substance was extracted with diethyl ether and dried usingmagnesium sulfate, and the solvent was removed by evaporation. Then,distillation was performed, whereby 1,3-dimethylpyrrolidine (13.3 g, 134mmol) which is a colorless transparent liquid was obtained.

Bromopropane (22.3 g, 182 mmol) was added to methylene chloride (10 ml)to which 1,3-dimethylpyrrolidine (12.0 g, 121 mmol) was added, and themixture was heated and refluxed for 24 hours. The solvent was removed byevaporation, and the obtained white residue was recrystallized inethanol and ethyl acetate and then dried under reduced pressure at 80°C. for 24 hours, whereby 1,3-dimethyl-1-propylpyrrolidinium bromide(13.9 g, 63.4 mmol) which is a white solid was obtained.

In pure water, 1,3-dimethyl-1-propylpyrrolidinium bromide (5.30 g, 23.9mmol) and lithium bis(trifluoromethanesulfonyl)amide (7.55 g, 26.3 mmol)were mixed and stirred, so that an ionic liquid which is insoluble inwater was obtained immediately. After that, the obtained ionic liquidwas extracated with methylene chloride and then washed with pure watersix times. The solvent was removed by evaporation and dried in vacuum at100° C., so that 1,3-dimethyl-1-propylpyrrolidiniumbis(trifluoromethanesulfonyl)amide (9.37 g, 22.2 mmol) was obtained.

The compound obtained through the above steps was identified as1,3-dimethyl-1-propylpyrrolidinium bis(trifluoromethanesulfonyl)amidewhich is a target substance by using a nuclear magnetic resonance (NMR)and mass spectrometry.

¹H NMR data of the obtained compound is shown below.

¹H-NMR (CDCl₃, 400 MHz, 298 K): δ=0.97-1.05 (3H), 1.15-1.21 (3H),1.67-1.99 (3H), 2.28-2.48 (1H), 2.58-2.78 (1H), 2.94-3.08 (1H), 3.06,3.13 (3H), 3.18-3.34 (2H), 3.47-3.87 (3H)

A ¹H NMR chart of the obtained compound is shown in FIG. 7.

The measurement results of the electro spray ionization massspectrometry (ESI-MS) of the obtained compound are shown below.

MS (ESI-MS): m/z=142.22 (M)⁺; C₉H₂₀N (142.16), m/z=279.95 (M)⁻;C₂F₆NO₄S₂ (279.92)

Next, physical property values of the obtained compound were obtained inthe following manner.

The viscosity was measured with an oscillation type viscometer (VM-10A)produced by SEKONIC CORPORATION at 25° C. Since a measurement valueobtained with the oscillation type viscometer is “viscosity(mPa·s)×density (g/ml)”, the viscosity is obtained by dividing themeasurement value by the density.

The conductivity was obtained by filling a conductivity cell, which is ahousing made of a fluorine resin and includes a plate electrode producedby SUS Corporation, with a sample and by alternating current impedancemeasurement.

The alternating current impedance measurement uses impedance measurementsystem composed of a potentiostat and a frequency response analyzer(FRA) to analyze a response current against a small voltage amplitudeapplied to an object to be measured.

The alternating current impedance was measured with an electrochemicalmeasurement system HZ-5000 produced by Hokuto Denko Corporationconnected with a frequency response analyzer FRA5022 produced by NFCorporation, under the conditions that the AC (alternating current)amplitude was 10 mV and the temperature was 25° C.

The melting point was measured with a differential scanning calorimeterDSC200 produced by SII NanoTechnology Inc., under the conditions thatthe temperature range was from −100° C. to 100° C. and the temperaturerising rate was 10° C./min.

The obtained compound had a viscosity of 68 mPa·s, a conductivity of 3.4mS/cm, and a melting point of −14° C.

Example 2

In this example, a method for producing1,3-dimethyl-1-propylpyrrolidinium bis(fluorosulfonyl)amide(abbreviation: 3mP13-FSA) represented by the structural formula (201) isdescribed.

First, 1,3-dimethyl-1-propylpyrrolidinium bromide was synthesized in amanner similar to that in Example 1.

In pure water, 1,3-dimethyl-1-propylpyrrolidinium bromide (11.4 g, 51.3mmol) and potassium bis(fluorosulfonyl)amide (12.3 g, 56.1 mmol) weremixed and stirred, so that an ionic liquid which is insoluble in waterwas obtained immediately. After that, the obtained ionic liquid wasextracated with methylene chloride and then washed with pure water sixtimes. The solvent was removed by evaporation and dried in vacuum at100° C., so that 1,3-dimethyl-1-propylpyrrolidiniumbis(fluorosulfonyl)amide (12.2 g, 37.8 mmol) was obtained.

The compound obtained through the above steps was identified as1,3-dimethyl-1-propylpyrrolidinium bis(fluorosulfonyl)amide which is atarget substance by using a nuclear magnetic resonance (NMR) and massspectrometry.

¹H NMR data of the obtained compound is shown below.

¹H-NMR (CDCl₃, 400 MHz, 298 K): δ=0.99-1.08 (3H), 1.17-1.24 (3H),1.74-1.97 (3H), 2.33-2.50 (1H), 2.60-2.78 (1H), 2.97-3.08 (1H), 3.09,3.16 (3H), 3.20-3.37 (2H), 3.48-3.88 (3H)

A ¹H NMR chart of the obtained compound is shown in FIG. 8.

The measurement results of the electron impact mass spectrometry (EI-MS)of the obtained compound are shown below

MS (EI-MS): m/z=142.23 (M)⁺; C₉H₂₀N (142.16), m/z=180.00 (M)⁻; F₂NO₄S₂(179.92)

The physical property values of the obtained compound were obtained by amethod similar to that in Example 1.

The obtained compound had a viscosity of 50 mPa·s, a conductivity of 6.4mS/cm, a glass-transition temperature of −101° C., and no observed clearmelting point.

Example 3

In this example, a method for producing1,2-dimethyl-1-propylpyrrolidinium bis(fluorosulfonyl)amide(abbreviation: 2mP13-FSA) represented by the structural formula (202) isdescribed.

First, 2-methylpyrrolidine (8.52 g, 100 mmol) was gradually added toformic acid (12.8 g, 250 mmol) being water-cooled. Next, a 37%formaldehyde solution (11 ml, 150 mmol) was added to this solution. Thissolution was heated and refluxed at 100° C., was cooled back to roomtemperature after a bubble generation, and was stirred for about 30minutes. Then, the solution was heated and refluxed again for one hour.

The formic acid was neutralized with sodium hydroxide, and then thetarget substance was extracted with dichloromethane and dried usingmagnesium sulfate, and the solvent was removed by evaporation. Then,distillation was performed, whereby 1,2-dimethylpyrrolidine (6.97 g,70.3 mmol) which is a colorless transparent liquid was obtained.

Bromopropane (12.9 g, 105 mmol) was added to methylene chloride (10 ml)to which 1,2-dimethylpyrrolidine (6.97 g, 70.3 mmol) was added, and themixture was heated and refluxed for 24 hours. The solvent was removed byevaporation, and the obtained white residue was recrystallized inethanol and ethyl acetate and then dried under reduced pressure at 80°C. for 24 hours, whereby 1,2-dimethyl-1-propylpyrrolidinium bromide(8.36 g, 37.6 mmol) which is a white solid was obtained.

In pure water, 1,2-dimethyl-1-propylpyrrolidinium bromide (6.44 g, 29.0mmol) and potassium bis(fluorosulfonyl)amide (6.99 g, 3.19 mmol) weremixed and stirred, so that an ionic liquid which is insoluble in waterwas obtained immediately. After that, the obtained ionic liquid wasextracated with methylene chloride and then washed with pure water sixtimes. The solvent was removed by evaporation and dried in vacuum at100° C., so that 1,2-dimethyl-1-propylpyrrolidiniumbis(fluorosulfonyl)amide (7.61 g, 23.6 mmol) was obtained.

The compound obtained through the above steps was identified as1,2-dimethyl-1-propylpyrrolidinium bis(fluorosulfonyl)amide which is atarget substance by using a nuclear magnetic resonance (NMR) and massspectrometry.

¹H NMR data of the obtained compound is shown below.

¹H-NMR (CDCl₃, 400 MHz, 298 K): δ=1.05-1.11 (3H), 1.44-1.47 (3H),1.75-1.96 (3H), 2.04-2.20 (1H), 2.22-2.35 (1H), 2.44-2.58 (1H), 2.81,3.09 (3H), 2.94-3.89 (5H)

A ¹H NMR chart of the obtained compound is shown in FIG. 9.

The measurement results of the electron impact mass spectrometry (EI-MS)of the obtained compound are shown below.

MS (EI-MS): m/z=142.22 (M)⁺; C₉H₂₀N (142.16), m/z=180.00 (M)⁻; F₂NO₄S₂(179.92)

The physical property values of the obtained compound were obtained by amethod similar to that in Example 1.

The viscosity of the obtained compound was 82 mPa·s, and theconductivity thereof was 3.6 mS/cm. In addition, the melting point was−34° C.

Example 4

In this example, the calculation results of potential windows of3mP13-TFSA in Example 1, 2mP13-FSA in Example 2, and 3mP13-FSA inExample 3 by linear sweep voltammetry are described.

Samples used in this example are described. As Sample 1, Sample 2, andSample 3, 3mP13-TFSA in Example 1, 2mP13-FSA in Example 2, and 3mP13-FSAin Example 3 were used, respectively. Further, as Comparative Sample 1,1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)amide (abbreviation:P13-FSA) produced by KANTO CHEMICAL CO., INC. was used, as ComparativeSample 2, 1-methyl-1-propylpyrrolidiniumbis(trifluoromethanesulfonyl)amide (abbreviation: P13-TFSA) produced byKishida Chemical Co., Ltd.) was used, and as Comparative Sample 3,1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide (abbreviation:EMI-FSA) produced by KANTO CHEMICAL CO., INC. was used.

The measurement was performed with electrochemical measurement systemHZ-5000 produced by HOKUTO DENKO CORPORATION in a glove box with anargon atmosphere. A glassy carbon electrode was used as a workingelectrode and a platinum wire was used for an opposite electrode. Asilver wire immersed in a solution in which silvertrifluoromethanesulfonate was dissolved in 1-ethyl-3-methylimidazoliumbis(trifluoromethanesulfonyl)amide at a concentration of 0.1 M was usedfor a reference electrode. Oxidation-reduction potential of the ionicliquid was corrected with reference to the oxidation-reduction potentialof ferrocene (Fc/Fc⁺). The potential scanning speed was 50 mV/s.

FIG. 10 shows linear sweep voltammograms of Samples 1 to 3 andComparative Samples 1 to 3. In FIG. 10, the heavy lines indicate Samples1 to 3 and the thin lines indicate Comparative Samples 1 to 3. The heavysolid line indicates Sample 1, the heavy dash line indicates Sample 2,and the heavy alternate long and short dash line indicates Sample 3. Thethin solid line indicates Comparative Sample 1, the thin dotted lineindicates Comparative Sample 2, and the thin alternate long and twoshort dashes line indicates Comparative Sample 3. Table 2 shows thereduction potentials, oxidation potentials, and potential windows ofSamples 1 to 3 and Comparative Samples 1 to 3. A “potential window” inthis example refers to a difference between an oxidation potential and areduction potential. In FIG. 10, a potential at which an electriccurrent density of −1 mA/cm² was detected during the scanning ofpotentials was calculated as a reduction potential. Further, in FIG. 10,a potential at which an electric current density of 1 mA/cm² wasdetected during the scanning of the potentials was calculated as anoxidation potential. The potential window was calculated by subtractinga “reduction potential” from an “oxidation potential”.

TABLE 2 Potential window Reduction Oxidation (v.s. Li/Li⁺) [mA · cm⁻²][mA · cm⁻²] [mA · cm⁻²] Sample 1 −0.1 5.7 5.8 Sample 2 0.1 5.8 5.7Sample 3 0.2 5.4 5.2 Comparative Sample 1 0.2 5.6 5.4 Comparative Sample2 0.0 5.7 5.7 Comparative Sample 3 0.9 5.2 4.3

Each of Samples 1 to 3, which is an ionic liquid according to anembodiment of the present invention, has a lower reduction potential anda higher oxidation potential than Comparative Sample 3 which is an ionicliquid including imidazolium-based cations. Furthermore, each of Samples1 to 3, which is an ionic liquid according to an embodiment of thepresent invention, has a comparable potential window to ComparativeSamples 1 and 2, each of which is an ionic liquid including cyclicquaternary ammonium-based cations.

Next, the measurement results of the solution resistances of the ionicliquids and the cell resistances of electric double layer capacitorseach using an ionic liquid for an electrolyte solution are describedwith reference to FIG. 11 and FIGS. 12A and 12B.

First, a method for manufacturing the electric double layer capacitor isdescribed with reference to FIG. 11.

The electric double layer capacitor manufactured in this example is alaminated electric double layer capacitor as illustrated in FIG. 11.

A laminated electric double layer capacitor 320 includes a positiveelectrode 303 including a positive electrode current collector 301 and apositive electrode active material layer 302, a negative electrode 306including a negative electrode current collector 304 and a negativeelectrode active material layer 305, a separator 307, an electrolytesolution 308, and a housing 309.

A method for manufacturing the laminated electric double layer capacitorused in this example is described.

In this example, commercially available objects were used as thecomponents except the electrolyte solution 308, i.e., the positiveelectrode 303, the negative electrode 306, the separator 307, and thehousing 309. Specifically, an electrode sold by TAKUMI GIKEN CORPORATIONwas used as the positive electrode 303, in which aluminum foil was usedfor the positive electrode current collector 301, and activecarbon/conductive material/styrene-butadiene rubber (SBR)/carboxy methylcellulose (CMC) (=90/10/2/2) was used for the positive electrode activematerial layer 302. Similarly, an electrode sold by TAKUMI GIKENCORPORATION was used as the negative electrode 306, in which aluminumfoil was used for the negative electrode current collector 304, andactive carbon/conductive material/SBR/CMC (=90/10/2/2) was used for thenegative electrode active material layer 305. As the separator 307,solvent-spun regenerated cellulosic fiber (TF40) produced by NIPPONKODOSHI CORPORATION was used. As the housing 309, a valved body made ofan aluminum laminated film produced by Hohsen Corp. (outer layer: a25-μm-thick nylon/40-μm-thick aluminum layer, inner layer: a22.5-μm-thick acid modified polypropylene/22.5-μm-polypropylene layer)was used. The separator 307 was interposed between the positiveelectrode 303 and the negative electrode 306, and they were put in thehousing 309. The housing 309 was filled with the electrolyte solution308 and then sealed.

Here, an electric double layer capacitor including 3mP13-FSA as theelectrolyte solution 308 was used as Sample 4, and an electric doublelayer capacitor including P13-FSA produced by KANTO CHEMICAL CO., INC.as the electrolyte solution 308 was used as Comparative Sample 4.

Next, the solution resistances and the cell resistances of Sample 4 andComparative Sample 4 were measured. The alternating current impedancemeasurement was performed for obtaining the solution resistances and thecell resistances.

The impedance of each electric double layer capacitor was measured whilethe temperatures of Sample 4 and Comparative Sample 4 are maintained at25° C., 20° C., 10° C., 0° C., −10° C., −20° C. −30° C., and −40° C. ina thermostatic bath produced by ESPEC Corp. Here, an AC impedancemeasurement at constant potential was conducted with amulti-potentiostat VSP produced by by HOKUTO DENKO CORPORATION. Themeasurement conditions were as follows: the initial frequency was 200kHz, AC amplitude was 10 mV. the last frequency was 20 mHz, and themeasurement was performed after 15-minute application of a voltage of1.25 V.

FIG. 12A shows the solution resistances of Sample 4 and ComparativeSample 4, and FIG. 12B shows the cell resistances of Sample 4 andComparative Sample 4. Note that the solution resistances shown in FIG.12A are obtained at a frequency of 200 kHz, and the cell resistancesshown in FIG. 12B are obtained at a frequency of 20 mHz. In FIGS. 12Aand 12B, the horizontal axis indicates temperature [° C.] and thevertical axis indicates impedance [Ω]. Also in FIGS. 12A and 12B,circles indicate Sample 4 and triangles indicate Comparative Sample 4.

As seen from FIGS. 12A and 12B, the resistance value of ComparativeSample 4 increases as the temperature is decreased, and the resistancevalue extremely increases when the temperature becomes −20° C. or lower.On the other hand, although the resistance value of Sample 4 alsoincreases as the temperature is decreased, the resistance value does notextremely increases.

The reason of the extreme increase in resistance value is probably thatphase transition from liquid to solid occurs in Comparative Sample 4when the temperature becomes −20° C. or lower; whereas, liquid does notchange to solid in Sample 4 as the temperature is decreased; thus, theresistance value did not increase extremely.

Next, the evaluation results of the load characteristics of an electricdouble layer capacitor using an ionic liquid for an electrolyte solutionare described with reference to FIG. 13.

The laminated electric double layer capacitor illustrated in FIG. 11 wasused for measuring the load characteristics. Further, electric doublelayer capacitors having the same structure as Sample 4 and ComparativeSample 4 were fabricated. Here, an electric double layer capacitorhaving the same structure as Sample 4 was used as Sample 5, and anelectric double layer capacitor having the same structure as ComparativeSample 4 was used as Comparative Sample 5. In addition, an electricdouble layer capacitor in which 1.0 M Et₄NBF₄/PC produced by KishidaChemical Co., Ltd. is used as the electrolyte solution 308 was used asComparative Sample 6. Note that Comparative Sample 6 has the samestructure as the other electric double layer capacitors except for theelectrolyte solution 308.

Next, Sample 5, Comparative Sample 5, and Comparative Sample 6 weresubjected to a charge/discharge test. The charge/discharge test wasperformed with a battery charge/discharge tester HJ-1010D8 produced byHOKUTO DENKO CORPORATION at 25° C. The charging consisted of CC chargingat 4 mA (10 C) followed by CV charging at 2.5 V (charge stop condition:0.4 mA). The discharging was performed at 0.4 mA to 400 mA (1 C to 1000C).

FIG. 13 shows output characteristics of Sample 5, Comparative Sample 5,and Comparative Sample 6. The horizontal axis indicates current densityand the vertical axis indicates capacitance. In FIG. 13, circlesindicate Sample 5, triangles indicate Comparative Sample 5, and squaresindicate Comparative Sample 6.

As shown in FIG. 13, it was found that Sample 5 using the ionic liquidaccording to an embodiment of the present invention for an electrolytesolution has comparable output characteristics to Comparative Sample 6using an organic solvent for an electrolyte solution.

From the results shown in FIG. 10, FIG. 11, FIGS. 12A and 12B, and FIG.13, the ionic liquid according to an embodiment of the present inventionhas a wide potential window, high electrochemical stability, and a lowmelting point.

Example 5

In this example, a method for producing1-ethyl-1,3-dimethylpyrrolidinium bis(fluorosulfonyl)amide(abbreviation: 3mP12-FSA) represented by the structural formula (203) isdescribed.

First, in an N₂ atmosphere, bromoethane (19.77 g, 182 mmol) was added todehydrated tetrahydrofuran (10 ml) to which 1,3-dimethylpyrrolidine(12.0 g, 121 mmol) was added, and the mixture was heated and refluxedfor 24 hours. Next, the solvent was removed by evaporation, and theobtained white residue was recrystallized in ethanol and ethyl acetateand then dried under reduced pressure at 80° C. for 24 hours, whereby1-ethyl-1,3-dimethylpyrrolidinium bromide (12.90 g, 62 mmol) which is awhite solid was obtained.

In pure water, 1-ethyl-1,3-dimethylpyrrolidinium bromide (12.90 g, 62mmol) and potassium bis(fluorosulfonyl)amide (10.08 g, 46 mmol) weremixed and stirred, so that an ionic liquid which is insoluble in waterwas obtained immediately. After that, the obtained ionic liquid wasextracated with methylene chloride and then washed with pure water sixtimes. The solvent was removed by evaporation and dried under reducedpressure at 60° C. for approximately eight hours, so that1-ethyl-1,3-dimethylpyrrolidinium bis(fluorosulfonyl)amide (16.26 g, 53mmol) was obtained.

The compound obtained through the above steps was identified as1-ethyl-1,3-dimethylpyrrolidinium bis(fluorosulfonyl)amide which is atarget substance by using a nuclear magnetic resonance (NMR) and massspectrometry.

¹H NMR data of the obtained compound is shown below.

¹H-NMR (CDCl₃, 400 MHz, 298 K): δ=1.15-1.25 (3H), 1.35-1.55 (3H),1.83-1.89 (1H), 2.31-2.50 (1H), 2.58-2.78 (1H), 2.94-3.09 (1H), 2.99,3.05, 3.09, 3.15 (3H), 3.31-3.62 (2H), 3.45-3.62 (2H), 3.62, 3.83 (1H)

A ¹H NMR chart of the obtained compound is shown in FIG. 14.

The measurement results of the electro spray ionization massspectrometry (ESI-MS) of the obtained compound are shown below.

MS (ELMS): m/z=142.23 (M)⁺; C₉H₂₀N (142.16), m/z=180.00 (M)⁻; F₂NO₄S₂(179.92)

The physical property values of the obtained compound were obtained by amethod similar to that in Example 1.

The viscosity of the obtained compound was 42 mPa·s, and theconductivity thereof was 8.9 mS/cm. In addition, the melting point was−26° C.

Next, the calculation result of a potential window of 3mP12-FSA bylinear sweep voltammetry is described.

The measurement was performed with electrochemical measurement systemHZ-5000 produced by HOKUTO DENKO CORPORATION in a glove box with anargon atmosphere. A glassy carbon electrode was used as a workingelectrode and a platinum wire was used for an opposite electrode. Asilver wire immersed in a solution in which silvertrifluoromethanesulfonate was dissolved in 1-ethyl-3-methylimidazoliumbis(trifluoromethanesulfonyl)amide at a concentration of 0.1 M was usedfor a reference electrode. Oxidation-reduction potential of the ionicliquid was corrected with reference to the oxidation-reduction potentialof ferrocene (Li/Li⁺). The potential scanning speed was 50 mV/s.

FIG. 15 shows a linear sweep voltammogram of 3mP12-FSA. In FIG. 15, apotential at which an electric current density of −1 mA/cm² was detectedduring the scanning of potentials was calculated as a reductionpotential. Further, in FIG. 15, a potential at which an electric currentdensity of 1 mA/cm² was detected during the scanning of the potentialswas calculated as an oxidation potential. The potential window wascalculated by subtracting a “reduction potential” from an “oxidationpotential”. From the results shown in FIG. 15, the reduction potentialwas 0.2 V, the oxidation potential was 5.6 V, and the potential windowwas 5.4 V. Consequently, 3mP12-FSA is an ionic liquid having a widepotential window.

Example 6

In this example, the results of a charge/discharge test of a lithium-ionsecondary battery according to an embodiment of the present inventionare described.

First, a method for manufacturing a lithium-ion secondary battery isdescribed with reference to FIG. 16.

The lithium-ion secondary battery manufactured in this example is acoin-type lithium-ion secondary battery as shown in FIG. 16.

A coin-type lithium-ion secondary battery 330 includes a positiveelectrode 333 including a positive electrode current collector 331 and apositive electrode active material layer 332, a negative electrode 336including a negative electrode current collector 334 and a negativeelectrode active material layer 335, a separator 337, and housings 339 aand 339 b.

A method for manufacturing the coin-type lithium-ion secondary batteryin this example is described.

In this example, commercially available objects were used as thecomponents except the electrolyte solution, i.e., the positive electrode333, the negative electrode 336, the separator 337, and the housings 339a and 339 b. Specifically, aluminum foil was used for the positiveelectrode current collector 331 and the positive electrode activematerial layer 332 including LiFePO₄/acetylene black/polyvinylidenedifluoride (PVDF) (=85/7/8) was formed over the positive electrodecurrent collector 331, so that the positive electrode 333 was formed. Alithium electrode was used as the negative electrode 336, and membranefilter (Durapore VVLP04700) produced by Merck Ltd., was used as theseparator 337.

In addition, 2032 type coin cells sold by Hohsen Corp. were used as thehousings 339 a and 339 b. The positive electrode 333 was put in thehousing 339 a, the electrolyte solution was injected into the housing,and the negative electrode 336 and the housing 339 b were stackedthereover. The housing 339 a and the housing 339 b were pressed andcrimped to each other with a “coin cell crimper”; thus, the coin-typelithium-ion secondary battery was manufactured.

Here, as Sample 6, a lithium-ion secondary battery for which anelectrolyte solution obtained by dissolving lithiumbis(trifluoromethanesulfonyl)amide (abbreviation: LiTFSA) ofapproximately 1 M in 3mP13-FSA is used was used; and as Sample 7, alithium-ion secondary battery for which an electrolyte solution obtainedby dissolving LiTFSA in 3mP12-FSA is used was used.

Sample 6 and Sample 7 were subjected to a charge/discharge test. Thecharge/discharge test was performed with a battery charge/dischargetester HJ-1010D8 produced by HOKUTO DENKO CORPORATION at −25° C. in athermostatic bath produced by ESPEC Corp. The charging consisted of CCcharging at 0.04 mA (0.02 C) to increase the voltage to 4.0 V. Thedischarging was performed at 0.04 mA to 0.2 mA (0.02 C to 0.1 C).

FIG. 17A shows output characteristics of Sample 6, and FIG. 17B showsoutput characteristics of Sample 7. The horizontal axis indicatescapacitance and the vertical axis indicates voltage.

As shown in FIGS. 17A and 17B, each of Sample 6 and Sample 7 in which anelectrolyte solution includes an ionic liquid according to an embodimentof the present invention, is a lithium-ion secondary battery which canbe charged/discharged at a low-temperature environment of −25° C.Therefore, an ionic liquid according to an embodiment of the presentinvention does not change the phase from liquid to solid at alow-temperature environment of −25° C.

This application is based on Japanese Patent Application serial no.2011-125116 filed with Japan Patent Office on Jun. 3, 2011, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. An ionic liquid represented by a general formula(G0),

wherein: one of R⁰ and R⁵ is any of a methoxy group, a methoxymethylgroup, and a methoxyethyl group; the other of R⁰ and R⁵ is an alkylgroup having 1 to 20 carbon atoms, or a hydrogen atom; one or two of R¹to R⁴ are any of an alkyl group having 1 to 20 carbon atoms, a methoxygroup, a methoxymethyl group, and a methoxyethyl group; the others of R¹to R⁴ are each a hydrogen atom; and A⁻ is a univalent imide-based anion,a univalent methide-based anion, a perfluoroalkyl sulfonic acid anion,tetrafluoroborate, or hexafluorophosphate.
 2. The ionic liquid accordingto claim 1, wherein A⁻ is any one of univalent anions selected from(C_(n)F_(2n+1)SO₂)₂N⁻ (n=0 to 4), (C_(m)F_(2m+1)SO₃)— (m=0 to 4), andCF₂(CF₂SO₂)₂N—.
 3. The ionic liquid according to claim 1, wherein theone or two of R¹ to R⁴ are each an alkyl group having 1 to 4 carbonatoms.
 4. The ionic liquid according to claim 1, wherein: R¹ and R² areindividually any of an alkyl group having 1 to 20 carbon atoms, amethoxy group, a methoxymethyl group, and a methoxyethyl group, and R³and R⁴ are each a hydrogen atom.
 5. The ionic liquid according to claim4, wherein R¹ and R² are individually an alkyl group having 1 to 4carbon atoms.
 6. The ionic liquid according to claim 1, wherein: R¹ isany of an alkyl group having 1 to 20 carbon atoms, a methoxy group, amethoxymethyl group, and a methyoxyethyl group, and R² to R⁴ are each ahydrogen atom.
 7. The ionic liquid according to claim 1, wherein: R² isany of an alkyl group having 1 to 20 carbon atoms, a methoxy group, amethoxymethyl group, and a methoxyethyl group, and R¹, R³ and R⁴ areeach a hydrogen atom.
 8. A power storage device comprising a positiveelectrode, a negative electrode, a separator, and an electrolytesolution comprising an ionic liquid represented by a general formula(G0),

wherein: one of R⁰ and R⁵ is any of a methoxy group, a methoxymethylgroup, and a methoxyethyl group; the other of R⁰ and R⁵ is an alkylgroup having 1 to 20 carbon atoms, or a hydrogen atom; one or two of R¹to R⁴ are any of an alkyl group having 1 to 20 carbon atoms, a methoxygroup, a methoxymethyl group, and a methoxyethyl group; the others of R¹to R⁴ are each a hydrogen atom; and A⁻ is a univalent imide-based anion,a univalent methide-based anion, a perfluoroalkyl sulfonic acid anion,tetrafluoroborate, or hexafluorophosphate.
 9. The power storage deviceaccording to claim 8, wherein A⁻ is any one of univalent anions selectedfrom (C_(n)F_(2n+1)SO₂)₂N⁻ (n=0 to 4), (C_(m)F_(2m+1)SO₃)— (m=0 to 4),and CF₂(CF₂SO₂)₂N—.
 10. The power storage device according to claim 8,wherein the one or two of R¹ to R⁴ are each an alkyl group having 1 to 4carbon atoms.
 11. The power storage device according to claim 8,wherein: R¹ and R² are individually any of an alkyl group having 1 to 20carbon atoms, a methoxy group, a methoxymethyl group, and a methoxyethylgroup, and R³ and R⁴ are each a hydrogen atom.
 12. The power storagedevice according to claim 11, wherein R¹ and R² are individually analkyl group having 1 to 4 carbon atoms.
 13. The power storage deviceaccording to claim 8, wherein: R¹ is any of an alkyl group having 1 to20 carbon atoms, a methoxy group, a methoxymethyl group, and amethyoxyethyl group, and R² to R⁴ are each a hydrogen atom.
 14. Thepower storage device according to claim 8, wherein: R² is any of analkyl group having 1 to 20 carbon atoms, a methoxy group, amethoxymethyl group, and a methoxyethyl group, and R¹, R³ and R⁴ areeach a hydrogen atom.
 15. The power storage device according to claim 8,wherein the electrolyte solution further comprises an electrolyte saltincluding a lithium ion.
 16. The power storage device according to claim8, wherein the power storage device is a lithium-ion capacitor or anelectric double layer capacitor.
 17. The power storage device accordingto claim 8, wherein the power storage device is a lithium-ion secondarybattery.